The present invention relates to improvements in the heat resistance of silicones. More specifically, it relates to improvements in the thermal oxidative stability of alpha,omega-bis(trimethylsiloxy)polydimethylsiloxane.
While silicones in general are known for their thermal oxidative stability, it is advantageous to make them more resistant. For example, it might be desired to use a silicone fluid at a temperature of 275.degree. C., in air. In the absence of additives or special precautions, the silicone fluid will gel in a few hours, e.g. 1 to 6 hours. By gel, it is meant to become a non-pourable solid or semisolid.
Many substances have been used as additives to improve the thermal oxidative stability of silicone fluids. For example, the patent literature discloses the following additives: amines, ethers, catechols, borates, alkali metal salts of fluorinated acetic acid, substituted phenols, lignin, phosphites, pyrogallols, ferrocene, ammonium halides, triazines, nitriles, thioaniline derivatives, and a large number of other additives.
U.S. Pat. No. 2,465,296, issued Mar. 22, 1949 to J. Swiss, discloses, as additives to polymeric silicon compounds, chelates of metals from every group in the Periodic Table. The best gel times found by Swiss were 2160 hours, at 250.degree. C., with 0.2% by weight additive.
U.S. Pat. No. 2,445,567, issued July 20, 1948 to J. Elliott, discloses iron, cobalt, nickel, or copper carboxylates which are added to liquid organosubstituted polysiloxanes. Addition of such a carboxylate at the 0.1% level resulted in a 300.degree. gel time of 380 hours.
U.S. Pat. No. 4,070,343, issued Jan. 24, 1978, to Kishimoto et al., discloses the use of the reaction products of cerium salts with alkali metal siloxanolates, in conjunction with a zirconium, titanium, or iron carboxylic acid salt or alkoxy compound. The two salts are used in organosiloxane polymers.
None of the above references discloses the use of three salts in conjunction. The use of three particular salts in conjunction has been found to be a substantially more effective thermal oxidative stability additive for alpha,omega-bis(trimethylsiloxy)polydimethylsiloxanes than equivalent amounts of either one salt alone or two salts in conjunction.